1. Field of the Invention
This invention relates to hatch covers for cargo carrying vessels such as the hopper of a railway covered hopper car.
2. Description of the Prior Art
Locking arrangements for lockingly sealing a hatch cover to a hatch opening or coaming of a vessel such as a lading hopper of a railway covered hopper car are old and well known. The lading in a given vessel may have properties such that excess pressure builds up inside the vessel. When the locking arrangement is released, the excess pressure may swing the hatch cover open with considerable force, endangering human operators and potentially damaging the apparatus. To prevent this, the locking arrangement may be designed to retain the hatch cover in an intermediate "blow-off" position where the hatch cover is free to move enough to vent pressure but restrained against larger movement.
U.S. Pat. Nos. 4,157,146, 4,388,873, and 4,441,431 show various arrangements for sealing a hatch cover and retaining it in a blow-off position. U.S. Pat. No. 4,157,146 discloses a plurality of clamps mounted about the hatch circumference, requiring a plurality of repeated operator functions to open the hatch. U.S. Pat. Nos. 4,388,873 and 4,441,431 disclose a hatch cover securable by a single cam lock in both sealed and blow-off positions, but the latch structure is relatively weak and in the event of mechanical failure, the hatch cover might blow open.